Place the bones your slow cooker. The bones should fill up about 3/4 of the slow-cooker.
Chop your vegetables and garlic, no need to peel. You can also add vegetable scraps and rinsed, crushed egg shells into your slow-cooker. You'll be straining these out before consuming the broth.
Fill the slow-cooker with water. Season with a generous amount of salt (about 1 teaspoon).
Add 2 tablespoons of apple cider vinegar (you won't notice the taste).
Cook on low and cook for 18-72 hours.
Strain the broth through cheesecloth or a strainer and cool. A good broth will usually have a layer of fat on the top, and will gelatinize when thoroughly cool. Remove the fat with a spoon and discard.